1. Field of Invention
The invention relates to a device for the forging of sleeve-shaped objects like steel pistons, a method for the production of sleeve-shaped forgings and forgings produced according to the method.
2. Description of Related Art
Sleeve-shaped forgings are often produced from forgings of iron alloys, primarily steels. Such forgings are at least partially hot forged. A very common application of this type of sleeve-shaped parts is that of pistons for combustion engines. Although the invention is explained the context of the production engine pistons, these types of sleeves are also used in many other applications so that the present invention is generally applicable to all sleeve-shaped components made of forgeable material.
In the prior art, pistons for combustion engines are forged in two steps, wherein a blank piston is inserted into a die having a pre-cavity to produce a semi-finished element in a first step. The semi-finished element is then definitively forged by another cavity into a final forging in a second step. The device for implementing this two step method includes a lower form mandrel that is surrounded by an external form ring or forging sleeve.
Because pistons require a high level of mechanical forming to fill the cavity parts, piston blanks are often worked with flash cushions. In this way, the long flow distance of the material to be forged is regularly distributed over the forming steps.
The prior art method having essentially two forging steps is accompanied by temperature changes (cooling) which result in increased resistance against deformation. Such deformation resistance sets narrow boundaries to the achievable piston skirt thickness and to the piston skirt length. However, today, because of many reasons, it is desirable to achieve minimal piston wall thickness levels by having the biggest piston skirt length possible. Such a piston shape results in a low weight, an improved thermal expansion capacity, and a long control inside the cylinder sleeve. This desired thin wall can be achieved together with a long piston skirt only by having a very deep cavity and is achievable only by additional working beyond the two forging steps.
A method of this type is energy-consuming because at least two forging stations must be provided. Additionally, measures had to be taken in order to avoid excessive cooling of the forged parts between the stations which required additional handling costs.
For these reasons, a more rapid and easier forging method for sleeve-shaped objects is needed, as well as suitable equipment to implement such a method.
A further problem relating to the forging of sleeve-shaped parts in forging devices having cavity is the lifetime of the tools. Deep cavities, long flow distances, high pressure inside the tools, and rapid cooling by application of lubricants, all limit the life time of the lower form mandrel and the external form ring, or forging sleeve. A replacement or a rework of the tool and its parts causes downtime during the production process which makes the execution of the method more costly. Tools are expensive and must be put available for production with minimum downtime caused by the need to repair or replace them.
As a consequence, the problem the present invention seeks to solve consists of avoiding the disadvantages of prior art forging methods for sleeve-shaped forging elements.